Polyethylene resine

ABSTRACT

The invention provides ethylene/α-olefin copolymers exhibiting improved environmental stress cracking resistance properties, and methods for the production of the copolymers in a single reactor by means of a bimetallic catalyst including a Ziegler component and a metallocene component.

1. FIELD OF THE INVENTION

[0001] The invention relates generally to polyethylene resins. Inparticular, the invention provides ethylene/α-olefin copolymersexhibiting improved environmental stress cracking resistance properties,and methods for the production of the copolymers in a single reactor bymeans of a bimetallic catalyst including a Ziegler component and ametallocene component.

2. BACKGROUND

[0002] Environmental stress cracking is the phenomenon whereby astressed resin develops brittle cracks when exposed to a fluid such as adetergent or an organic liquid. This phenomenon can cause prematurefailure of articles manufactured from the resin. Environmental stresscracking resistance (“ESCR”) tests have been developed to measure theresistance of resins to their environment. One such test is described inASTM D1693. ESCR is commercially important particularly when resins comeinto contact with detergents and organic chemicals, such as householdchemical containers and organic chemical containers.

[0003] ESCR testing can also be used as a measure of a resin'sresistance to slow crack propagation. Slow crack propagation occurs inresins that are at low stress levels, over extended periods of time. Inthis case a brittle crack propagates through the materials. This type offailure mechanism is seen in commercial applications of polyethylene inpressure pipe, containers, and vessels. Commercial polyethylene pressurepipe systems are designed to have a lifetime in excess of fifty years.Improved ESCR at high stiffness would be particularly desirable for suchapplications.

[0004] It is known in the art that lowering resin density of linearpolyethylene resins, such as linear low density polyethylene (“LLDPE”),medium density polyethylene (“MDPE”) and high density polyethylene(“HDPE”), greatly improves the ESCR of the resins. However, thisimprovement in ESCR is at the expense of resin stiffness. As a result;conventional single reactor resins have a poor balance of ESCR and resinstiffness.

[0005] Resins with a bimodal molecular weight distribution, also termed“bimodal resins,” are resins having at least two polymer components withdifferent average molecular weights. In this description, the resin withthe higher average molecular weight is referred to as the “HMW polymercomponent”, and the resin with the lower average molecular weight isreferred to as the “LMW polymer component”. Resins with a bimodalmolecular weight distribution (“MWD”) can be produced in a singlereactor using the technology disclosed in, for example, U.S. Pat. No.5,539,076, or by the use of a series of reactors or reaction steps. Forexample, bimodal MWD polyethylene resins can be produced in a tandemslurry processes. Bimodal resins such as those produced in seriesreactors are known to have a good combination of high ESCR andstiffness, believed to be because the polymerization process iscontrolled to ensure that the comonomer is incorporated in the HMWpolymer component. U.S. Pat. No. 4,461,783 to Baily et al. disclosesthat high ESCR, high density resins may be obtained with independentlyprepared, mechanically blended polyethylene resins of different MWDwhere the HMW polymer component contains the majority of the comonomer,and the LMW polymer component is essentially a homopolymer.

[0006] U.S. Pat. No. 5,539,076 to Nowlin et al. discloses the productionof polyethylene resins with bimodal MWD in a single reactor using Ti/Zrbimetallic catalyst systems. However, in these resins, the comonomer ispredominantly in the LMW polymer component of the bimodal resin. Thistype of comonomer distribution does not meet the requirements asdisclosed in U.S. Pat. No. 4,461,783 for high ESCR at high resindensity. Other background references include WO 00/50466, WO 98/57998,WO 99/31146, U.S. Pat. No. 5,624,877 to Bergmeister et al., EP 0 619 325A1, and EP 0 882 744 A1.

3. SUMMARY OF THE INVENTION

[0007] It has now surprisingly been found that despite the expectedunfavorable branching distribution in ethylene/α-olefin copolymers withbimodal MWD produced by bimetallic (e.g., Ti/Zr) catalysts in a singlereactor, it is possible to produce such resins which exhibit very highESCR at high resin density. This unexpected result makes it possible toproduce polyethylene resins with a superior balance of density,stiffness, ESCR and fracture toughness, in a single reactor. The ESCR ofthese resins is better than that of comparable commercial materials withsimilar resin density, and better than that of resins previously madewith bimetallic catalysts, including those described in U.S. Pat. No.5,539,076.

[0008] Accordingly, the present invention generally relates to anethylene/α-olefin copolymer having a density of at least 0.953 g/cm³ anda Bent Strip ESCR, T₅₀, of at least 175 hours, the copolymer prepared ina single reactor. The copolymer generally will have a density of atleast 0.955 g/cm³, at least 0.957 g/cm³, at least 0.959 g/cm³ or atleast 0.960 g/cm³, with the T₅₀ being generally at least 200 hours, atleast 250 hours, least 300 hours, at least 350 hours, or at least 400hours.

[0009] The copolymer may have a Melt Flow Rate I₂₁, determined accordingto ASTM D-1238, at 190° C. and 21.6 kg, of at least 20 g/10 min., atleast about 22 g/10 min., or at least 24 g/10 min.

[0010] According to a further embodiment the copolymer has a Melt FlowRatio, 121/12 of at least 100 or at least 120, with the Melt Index, 12,being determined according to ASTM D-1238, at 190° C. and 2.16 kg.

[0011] In another aspect, the ethylene/α-olefin copolymer of the presentinvention has a bimodal molecular weight distribution and includes a HMWpolymer component and a LMW polymer component which has a lower averagemolecular weight (weight average molecular weight, M_(w), determined byGel Permeation Chromatography) and a higher density than the HMW polymercomponent, the copolymer prepared in a single reactor with apolymerization catalyst including a Ziegler component and a metallocenecomponent.

[0012] The density of the HMW polymer component generally will rangefrom 0.930 g/cm³ to 0.950 g/cm³. The density of the LMW polymercomponent can be at least 0.955 g/cm³.

[0013] The total density of the copolymer can be at least 0.954 g/cm³.

[0014] In another embodiment, the HMW polymer component has a molecularweight distribution, weight average molecular weight/number averagemolecular weight M_(w)/M_(n), of from 3 to 6 and the LMW polymercomponent has a M_(w)/M_(n) of not higher than 6.

[0015] It may be particularly advantageous for the weight ratio of HMWpolymer component to LMW polymer component to range from 65:35 to 35:65,or from 60:40 to 40:60.

[0016] In a further embodiment, the copolymer includes units derivedfrom one or more α-olefins containing 3 to 10 carbon atoms, or 4 to 8carbon atoms, such as 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexeneor 1-octene.

[0017] The copolymer will generally include 0.5 to 20 mol %, or 1 to 10mol %, of units derived from one or more α-olefins.

[0018] A further aspect of the present invention is an ethylene/α-olefincopolymer prepared in a single reactor and having a bimodal MWD and adensity of at least 0.953 g/cm³, the copolymer including a HMW polymercomponent and a LMW polymer component having a lower average molecularweight than the HMW polymer component, the HMW polymer componentincluding at least 30 mol % or at least 35 mol %, of the total α-olefinpresent in the copolymer.

[0019] The present invention also generally relates to a process formaking an ethylene/α-olefin copolymer as described above, in a singlereactor. The process includes contacting, under polymerizationconditions, ethylene, one or more α-olefins, hydrogen and apolymerization catalyst having a Ziegler component and a metallocenecomponent, the combination of Ziegler component and metallocenecomponent being selected to form a copolymer which has a HMW polymercomponent and a LMW polymer component, the HMW polymer componentincluding at least 30 mol % of the total α-olefin incorporated into thecopolymer. In a particular aspect, the copolymer is a copolymer ofethylene and 1-hexene.

[0020] The process may advantageously be carried out in a gas phasereactor or in a slurry reactor, although other reactors are alsosuitable.

[0021] In a still further aspect, the present invention relates to amethod of improving the ESCR of an ethylene/α-olefin copolymer producedin a single reactor and having a bimodal MWD. According to this method,the comonomers are polymerized in the presence of a polymerizationcatalyst including a Ziegler component and a metallocene component,which affords a copolymer having a HMW component and a LMW component,the HMW component including at least 30 mol % of the total α-olefinincorporated into the copolymer.

[0022] The above copolymers may, for example, be made into blow-moldedarticles, such as bottles, or into extruded articles, such as pipes.

[0023] A still further aspect of the present invention is apolymerization catalyst for the preparation, in a single reactor, of theabove-mentioned ethylene/α-olefin copolymers. The catalyst includes aZiegler component producing the HMW polymer component, and a metallocenecomponent producing the LMW polymer component, the metallocene componentincluding two cyclopentadiene rings which have a total of at least 3 andnot more than 8, or al least 4 and not more than 6, substitutions. Thesubstituents can be alkyl groups, particularly those having 1 to 4carbon atoms, and preferably methyl and/or ethyl groups.

[0024] In a particular embodiment, the metallocene is a zirconocene,with the Ziegler component generally including titanium and/or vanadium.

[0025] Specific metallocene components includebis(1,3-dialkylcyclopentadienyl)zirconium dichloride or dimethyl, andbis(1,3-dimethylcyclopentadienyl)zirconium dichloride. Specific Zieglercomponents include both magnesium and titanium.

[0026] A still further aspect of the present invention is a supportedbimetallic catalyst suitable for use in the production of theethylene/α-olefin copolymers described herein, the catalyst including asolid support, at least one non-metallocene transition metal source, atleast one metallocene compound, and at least one aluminoxane, the atleast one metallocene compound including at least one dicyclopentadienyltransition metal compound wherein each of the two cyclopentadienyl ringsis independently substituted by up to 4 or up to 5 substituents havingnot more than 4 carbon atoms, provided that two adjacent substituents onthe same ring together with the carbon atoms to which they are bondedmay form a 5- or 6-member non-aromatic ring and two substituents ondifferent rings may be replaced by a C₂-C₄ alkylene or alkylidene groupor a silicon-containing group which form a bridge between the rings andfurther provided that the total number of substituents on the rings doesnot exceed 6 or alternatively does not exceed 8.

[0027] In a particular aspect, the non-metallocene transition metalsource includes at least one Group IV or V transition metal, such astitanium, and also includes halogen, such as chlorine.

[0028] The support can include comprise silica, although many othersupport materials are also suitable, such as alumina and silica-alumina.

4. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention is further described in the detaileddescription which follows, in reference to the drawings by way ofnon-limiting examples of exemplary embodiments. In the drawings:

[0030]FIG. 1 shows Density versus Bent Strip ESCR for commercial resinsand resins according to the present invention;

[0031]FIG. 2 shows Branching Content (B.C.) as a function of MolecularWeight (Mw) for a bimodal MWD resin produced using Ti/Zr bimetalliccatalyst technology described in U.S. Pat. No. 5,539,076; and

[0032]FIG. 3 shows Branching Content (B.C.) as a function of MolecularWeight (Mw) for a bimodal MWD resin produced according to PolymerizationExample 1 herein.

5. DETAILED DESCRIPTION

[0033] 5.1 Catalyst

[0034] A preferred synthesis of the bimetallic catalyst for making thecopolymers of the present invention includes two stages: synthesis of asupported catalyst intermediate (preferably in the given order), andsynthesis of the final supported catalyst. The synthesis is preferablycarried out in a series of several consecutive steps under inertconditions in the substantial absence of water and molecular oxygen.

[0035] According to a preferred synthesis, support material is firstslurried in a non-polar solvent. Support materials for preparing thecatalysts of the present invention include solid, particulate, porousmaterials and may include support materials disclosed in U.S. Pat. No.4,173,547. Such support materials include, but are not limited to, metaloxides, hydroxides, halides or other metal salts, such as sulfates,carbonates, phosphates, silicates, and combinations thereof, and may beamorphous and/or crystalline. Some preferred support materials includesilica, alumina and combinations thereof. Support material particles mayhave any shape, and are preferably approximately spherical (such asobtainable, for example, by spray-drying).

[0036] Preferred support materials include particles, the optimum sizeof which can easily be established by one of ordinary skill in the art.A support material that is too coarse may lead to unfavorable results,such as low bulk density of the resulting polymer powder. Thus,preferred support materials include particles with average size(diameter) of less than 250 μm, or less than 200 μm, or less than 80 μm.Preferred support materials include particles larger than 0.1 μm, orlarger than 10 μm, because smaller silica particles may produce smallpolymer particles (fines) which can cause reactor instability.

[0037] Support material is preferably porous, as porosity increases thesurface area of the support material, which, in turn, provides moresites for reaction. The specific surface areas may be measured inaccordance with British Standards BS 4359, volume 1 (1969). The specificsurface area of support material used in accordance with the presentinvention is preferably at least 3 m²/g, at least 50 m²/g, at least 150m²/g, or at least 300 m²/g. There is no preferred upper limit to supportmaterial specific surface area. The specific surface area of supportmaterial is generally less than 1500 m²/g. The internal porosity ofsupport material may be measured as the ratio of the pore volume and theweight of the material, and can be determined by the BET technique asdefined and described by Brunauer et al., J. Am. Chem. Soc., 60, 209-319(1938). The internal porosity of support material is preferably largerthan 0.2 cm³/g, or larger than 0.6 cm³/g. There is no preferred upperlimit to support material internal porosity, which, as a practicalmatter, is limited by particle size and internal pore diameter. Thus,internal porosity is generally less than 2.0 cm³/g.

[0038] Preferred support materials for use in the present inventioninclude silica, particularly amorphous silica, such as high surface areaamorphous silica. Such support materials are commercially available froma number of sources, and include materials marketed under the tradenamesof Davison 952 or Davison 955 by the Davison Chemical Division of W.R.Grace and Company, or Crosfield ES70 by Crosfield Limited (surfacearea=300 m²¹ g; pore volume 1.65 cm³/g). The silica is in the form ofspherical particles, which are obtained by a spray-drying process. Asprocured, theses silicas are not calcined (dehydrated).

[0039] Because organometallic compounds used in the preparation in thebimetallic catalyst of the present invention may react with water, thesupport material is preferably substantially dry. Water that isphysically bound to the support material can be removed, such as bycalcination, prior to forming a bimetallic catalyst according to thepresent invention.

[0040] Preferred calcined support materials include support materialthat has been calcined at a temperature greater than 100° C., greaterthan 150° C., greater than 200° C., or greater than 250° C. As sinteringof the support material is preferably avoided, calcination is preferablyeffected at a temperature that is below the sintering temperature of thesupport material. Calcination of a support material, e.g., silica, isconveniently carried out at a temperature of not higher than 850° C., brnot higher than 650° C. Exemplary calcination temperatures are 300° C.,600° C., or 800° C. Total calcination times usually are not shorter than4 or 6 hours, whereas calcination times longer than 12 hours offer noparticular advantage.

[0041] Calcination of support material can be performed using anyprocedure known in the art, and the present invention is not limited bythe calcination method. A preferred method of calcination is disclosedin T. E. Nowlin et al., “Ziegler-Natta Catalysts on Silica for EthylenePolymerization,” J. Polym. Sci., Part A: Polymer Chemistry, Vol. 29,1167-1173 (1991).

[0042] Support materials used in the Examples herein can be prepared asfollows. In a fluidized-bed, silica (such as Davison 955), is heated insteps from ambient temperature to the desired calcining temperature(typically 600° C.). The silica is maintained at this temperature for 4to 6 hours, then allowed to cool to room temperature. The calcinationtemperature primarily affects the number of OH groups on the supportsurface. The number of OH groups on the support surface (silanol groupsin the case of silica) is approximately inversely proportional to thetemperature of drying or dehydration: the higher the temperature thelower the hydroxyl group content. In other words, at each calcinationtemperature, the support reaches a particular OH concentration, afterwhich additional heating has no further effect on the OH concentration.

[0043] The slurry of the support material in the non-polar solvent isprepared by introducing the support material into the solvent,preferably while stirring, and heating the mixture to 25 to 70° C.,preferably to 40 to 60° C. The most suitable non-polar solvents arematerials which are liquid at reaction temperatures and in which all ofthe reactants used later during the catalyst preparation are at leastpartially soluble. Preferred non-polar solvents are alkanes,particularly those containing 5 to 10 carbon atoms, such as isopentane,hexane, isohexane, n-heptane, isoheptane, octane, nonane, and decane.

[0044] Prior to use, the non-polar solvent should be purified to removetraces of water, molecular oxygen, polar compounds, and other materialscapable of adversely affecting catalyst activity. The temperature of theslurry before addition of the non-metallocene transition metal compoundshould not be in excess of 90° C., since otherwise a deactivation of thetransition metal component is likely to result. Accordingly, allcatalyst synthesis steps are preferably carried out at a temperaturebelow 90° C., more preferably below 80° C.

[0045] Following the preparation of a slurry of the support material ina non-polar solvent, the slurry is preferably contacted with anorganomagnesium compound.

[0046] Preferred organomagnesium compounds for use in the preparation ofthe present catalyst include dialkylmagnesium compounds of the generalformula (I):

R¹ _(m)MgR² _(n)  (I)

[0047] where R¹ and R² are the same or different branched or unbranchedalkyl groups containing 2 to 12 carbon atoms, preferably 4 to 8 carbonatoms, and m and n are each 0, 1 or 2, provided that the sum (m+n) isequal to the valence of Mg. A particular dialkylmagnesium compound isdibutylmagnesium.

[0048] The organomagnesium compound is believed to increase the activityof the catalyst; see, e.g., Nowlin et al., J. Polym. Sci.: Part A:Polymer Chemistry, Vol. 29, 1167-1173 (1991). The amount oforganomagnesium compound will generally be greater than 0.3 mmol/g,greater than 0.5 mmol/g, or greater than 0.7 mmol/g, where the amount oforganomagnesium compound is given as mmol magnesium per gram of supportmaterial. In the synthesis of the present catalyst, it is desirable toadd no more organomagnesium compound than will be deposited, physicallyor chemically, into the support, since any excess of the organomagnesiumcompound in the liquid phase may react with other chemicals used for thecatalyst synthesis and precipitate them outside of the support. Thedrying temperature of the support materials affects the number of siteson the support available for the dialkylmagnesium compound: the higherthe drying temperature the lower the number of sites. Thus, the exactratio of organomagnesium compound to support will vary and should bedetermined on a case-by-case basis to assure that preferably only somuch of the organomagnesium compound is added to the slurry as will bedeposited into the support without leaving excess organomagnesiumcompound in the liquid phase. Thus the ratios given below are intendedonly as an approximate guideline and the exact amount of organomagnesiumcompound is to be controlled by the functional limitation discussedabove; i.e., it should preferably not be greater than that which cancompletely be deposited into the support. The appropriate amount of theorganomagnesium compound can be determined in any conventional manner,such as by adding the organomagnesium compound to the slurry of thesupport material until free organomagnesium compound is detected in theliquid phase (for example, by taking a sample of the liquid phase andanalyzing it for Mg by one of several analytical procedures known in theart). If organomagnesium compound is added in excess of the amountdeposited into the support material, it can be removed by filtration andwashing of the support material. However, this is less desirable thanthe embodiment described above.

[0049] For example, for the silica support heated at about 600° C., theamount of the organomagnesium compound added to the slurry willgenerally be less than 1.7 mmol/g, less than 1.4 mmol/g, or less thanabout 1.1 mmol/g.

[0050] The treatment of the support material with the organomagnesiumcompound can in principle be carried out at any temperature at which theorganomagnesium compound is stable. The contacting of the slurry of thesupport material in a non-polar solvent with the organomagnesiumcompound will generally be carried out at a temperature between 20° C.and 80° C. Preferably the addition is carried out at slightly elevatedtemperature, such as at least 30° C., or at least 40° C. After theaddition of the organomagnesium compound is complete, the slurry willusually be stirred, preferably at the temperature of addition, for asufficient time to allow the organomagnesium compound to react and/orinteract with the support material substantially completely. Generally,this time will be not less than 0.1 hours or not less than 0.5 hours,although stirring for more than 2.0 hours will not bring about anysignificant further reaction/interaction.

[0051] Next, the support treated with the organomagnesium compound canbe contacted with a modifier compound. As modifier compound, variousclasses of compounds are suitable, although frequently alcohols such as1-butanol are used. A further example of a particularly advantageousmodifier compound is triethylsilanol. The modifier compound may be usedto modify the non-metallocene transition metal of the Ziegler componentof the catalyst of the present invention. Because the non-metallocenetransition metal Ziegler catalyst component produces the HMW polymercomponent of the polyethylene resin with a bimodal MWD, the modifiercompound has a direct effect on the polymer properties of the HMWpolymer component. Different modifier compounds afford different results(to a certain extent) with regard to the weight fraction, the averagemolecular weight and the MWD of the HMW polymer component. Thesedifferent properties can readily be established by one skilled in theart.

[0052] The amount of modifier compound used is sufficient to reactsubstantially completely with the organomagnesium/support intermediatematerial formed after the addition of the organomagnesium compound tothe slurried support material. Generally, the molar ratio oforganomagnesium compound, such as dialkylmagnesium compound, to modifiercompound will be at least 1:5, or at least 1:2, or at least 1:1, and nothigher than 15:1, or not higher than 10:1, or not higher than 6:1, ornot higher than 2:1.

[0053] Regarding the temperature at which the modifier compound is addedto the slurry of support material treated with the organomagnesiumcompound, there are no particular restrictions besides the thermalstability of the materials involved. Generally, the addition will becarried out at a temperature between room temperature (20° C.) and theboiling point of the non-polar solvent of the slurry. As a matter ofconvenience, the temperature can be about the same as that at which theorganomagnesium compound was added and at which the slurry oforganomagnesium compound-treated support material was stirred before theaddition of the modifier compound, respectively. Following addition ofthe modifier compound, the slurry will generally be stirred, at aboutthe temperature of addition, for a time period that is sufficient toallow the modifier compound to substantially completely react/interactwith the organomagnesium compound-treated support material. The stirringtime is generally at least 0.5 hours, or at least 1.0 hour.

[0054] After the reaction/interaction with the modifier compound, theresulting slurry of support material is contacted with one or morenon-metallocene transition metal compound (source). During this step,the slurry temperature is preferably maintained at 25 to 70° C.,particularly 40 to 60° C. As noted above, temperatures in the slurry of90° C. or greater are likely to result in deactivation of thenon-metallocene transition metal source. Suitable transition metalcompounds used herein include those of elements of Groups 4 and 5 of thePeriodic Table, particularly titanium-containing and vanadium-containingcompounds, provided that these compounds are at least somewhat solublein non-polar solvents. Non-limiting examples of such compounds aretitanium and vanadium halides, e.g., titanium tetrachloride, vanadiumtetrachloride, vanadium oxytrichloride, and titanium and vanadiumalkoxides, wherein the alkoxide moiety has a branched or unbranchedalkyl radical of 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms,and even more preferably 1 to 6 carbon atoms, such as methoxy, ethoxy,propoxy, isopropoxy, butoxy, pentoxy and hexoxy. Combinations of thesecompounds may also be used. The preferred transition metal compounds aretitanium-containing compounds, particularly tetravalenttitanium-containing compounds, such as TiCI₄.

[0055] The amount of non-metallocene transition metal compound employedis at least in part determined by the desired ratio of HMW polymercomponent to LMW polymer component in the ethylene/α-olefin copolymerwith a bimodal molecular weight distribution to be produced with thebimetallic catalyst. Because the non-metallocene transition metal(Ziegler) catalyst component will produce the HMW polymer component andthe metallocene catalyst component will produce the LMW polymercomponent, under otherwise identical polymerization conditions the ratioof HMW polymer component to LMW polymer component in the resultingcopolymer will increase with increasing molar ratio of non-metallocenetransition metal compound to metallocene compound employed for thepreparation of the catalyst. The total amount of catalyst components, onthe other hand, is limited by the capability of the specific supportmaterial used to accommodate the catalyst components. Generally,however, the non-metallocene transition metal is used in an amount thatresults in an atomic ratio of Mg of the organomagnesium compound (suchas a dialkylmagnesium compound used to treat the support) to transitionmetal(s) in the non-metallocene transition metal compound of at least0.5:1, or at least 1:1, or at least 1.7:1, and not higher than 5: 1, ornot higher than 3: 1, or not higher than 2:1.

[0056] Mixtures of non-metallocene transition metal compounds can alsobe used, and generally, no restrictions are imposed on thenon-metallocene transition metal compounds which can be included. Anynon-metallocene transition metal compound that can be used alone canalso be used in conjunction with other non-metallocene transition metalcompounds.

[0057] After addition of the non-metallocene transition metal compoundis complete, the slurry solvent is generally removed by evaporationand/or filtration, to obtain a free-flowing powder of a catalystintermediate.

[0058] Next, incorporation of the metallocene compound can beundertaken. The metallocene compound is preferably activated with analuminoxane.

[0059] Preferred metallocene compounds for use in the present inventionhave the general formula (II):

Cp₂MA₂  (II)

[0060] wherein M is titanium, zirconium or hafnium; Cp represents mono-or polysubstituted cyclopentadienyl, unsubstituted, mono- orpolysubstituted cyclopentadienyl that is part of a (preferablynon-aromatic) bicyclic or tricyclic moiety or the cyclopentadienylmoieties may be linked by a bridging group; and A represents halogenatom, hydrogen atom or hydrocarbon group.

[0061] In formula (II), the preferred transition metal atom M iszirconium. The substituents on the cyclopentadienyl group, will usuallybe (preferably straight-chain) alkyl groups having 1 to 6, or 1 to 4carbon atoms, such as methyl, ethyl, propyl or n-butyl. Thecyclopentadienyl group can also be part of an optionally substitutedbicyclic or tricyclic moiety such as tetrahydroindenyl or a partiallyhydrogenated fluorenyl group. The cyclopentadienyl groups can also bebridged, for example, by polymethylene or dialkylsilyl groups, such as—CH₂—, —CH₂—CH₂—, —CR′R″— and —CR′R″—CR′R″—where R′ and R″ are lower(C₁-C₄) alkyl groups or hydrogen atoms, —Si(CH₃)₂—,—Si(CH₃)₂—CH₂—CH₂—Si(CH₃)₂— or similar bridge groups. If A in formula(II) represents halogen, it represents F, Cl, Br and/or I and ispreferably chlorine. If A represents an alkyl group, the alkyl grouppreferably is a straight-chain or branched alkyl group containing 1 to 8carbon atoms, such as methyl, ethyl, propyl, isopropyl, n-butyl,isobutyl, n-pentyl, n-hexyl or n-octyl. Of course, the groups Cp may bethe same or different, but preferably they are the same. The sameapplies with respect to the groups A.

[0062] The cyclopentadienyl rings are substituted by a total of at least3, or at least 4 substituents, up to a total of 8 substituents, such as4 on each ring. Preferably both rings have the same number ofsubstituents. Without wishing to be bound by theory, it is speculatedthat the steric hindrance between the substituted cyclopentadienyl ringsresults in a positioning thereof, relative to the catalytic center M,which in comparison to the positioning of two unsubstituted ormonosubstituted cyclopentadienyl rings makes it harder for the α-olefincomonomer to reach the catalytic center. This, in turn, reduces the rateat which α-olefin comonomer molecules are incorporated into thecopolymer chain, leaving more molecules of α-olefin comonomer(s) forreaction at the other catalytically active center, the non-metallocenetransition metal of the Ziegler component. Thereby a more uniformdistribution of α-olefin comonomer(s) between the HMW polymer component(produced by the Ziegler catalyst component) and the LMW polymercomponent (produced by the metallocene catalyst component) can beachieved. If the steric hindrance around the metal of the metallocenecatalyst component becomes too great due to excessive substitution ofthe cyclopentadienyl rings, the catalytic activity of the metallocenecomponent will significantly decrease. This can result in too low anamount and/or too low a molecular weight of the LMW component producedby the metallocene component of the bimetallic catalyst. Factors thatdetermine a suitable upper limit of the total number of substituents(above 3) include, inter alia, the size of the substituents, theirrelative positions on the ring (e.g., 1,2 or 1,3), the size of thegroups A, the size of M and the size of the α-olefin comonomer(s) aswell as the activity of the Ziegler catalyst component used incombination with the metallocene component. Based on theseconsiderations, suitable metallocene components for a specific case canreadily be determined by one skilled in the art.

[0063] Particularly suitable metallocene compounds for use in thepreparation of the bimetallic catalyst of the present invention includebis(substituted cyclopentadienyl)metal dihalides, bis(substitutedcyclopentadienyl)metal hydridohalides, bis(substitutedcyclopentadienyl)metal monoalkyl monohalides, and bis(substitutedcyclopentadienyl)metal dialkyls wherein the metal is preferablyzirconium or hafnium, the halide groups are preferably chlorine and thealkyl groups (including cycloalkyl groups) preferably have 1 to 6 carbonatoms. Illustrative, non-limiting examples of corresponding metallocenesinclude:

[0064] bis(4,5,6,7-tetrahydroindenyl)zirconium dichloride;

[0065] ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride;

[0066] dimethylsilylbis(methylcyclopentadienyl)zirconium dichloride;

[0067] dimethylsilylbis(dimethylcyclopentadienyl)zirconium dichloride;

[0068] dimethylsilylbis(trimethylcyclopentadienyl)zirconium dichloride;

[0069] dimethylsilylbis(4,5,6,7-tetrahydroindenyl)zirconium dichloride;

[0070] bis(dimethylcyclopentadienyl)zirconium dibromide;

[0071] bis(dimethylcyclopentadienyl)methylzirconium chloride;

[0072] bis(dimethylcyclopentadienyl)ethylzirconium chloride;

[0073] bis(dimethylcyclopentadienyl)cyclohexylzirconium chloride;

[0074] bis(dimethylcyclopentadienyl)phenylzirconium chloride;

[0075] bis(dimethylcyclopentadienyl)benzylzirconium chloride;

[0076] bis(dimethylcyclopentadienyl)zirconium chloride monohydride;

[0077] bis(dimethylcyclopentadienyl)hafnium chloride monohydride;

[0078] bis(dimethylcyclopentadienyl)methylzirconium hydride;bis(dimethylcyclopentadienyl)dimethylzirconium;

[0079] bis(dimethylcyclopentadienyl)dimethylhafnium;

[0080] bis(dimethylcyclopentadienyl)diphenylzirconium;

[0081] bis(dimethylcyclopentadienyl)dibenzylzirconium;

[0082] bis(dimethylcyclopentadienyl)methoxyzirconium chloride;

[0083] bis(dimethylcyclopentadienyl)ethoxyzirconium chloride;

[0084] bis(dimethylcyclopentadienyl)zirconium bis(methanesulfonate);

[0085] bis(dimethylcyclopentadienyl)zirconium bis(p-toluenesulfonate);

[0086] bis(dimethylcyclopentadienyl)zirconiumbis(trifluoromethanesulfonate);

[0087] bis(diethylcyclopentadienyl)zirconium dichloride;

[0088] bis(dimethylcyclopentadienyl)zirconium dichloride;

[0089] bis(trimethylcyclopentadienyl)zirconium dichloride;

[0090] bis(tetramethylcyclopentadienyl)zirconium dichloride;

[0091] bis(methylethylcyclopentadienyl)zirconium dichloride;

[0092] bis(dipropylcyclopentadienyl)zirconium dichloride;

[0093] bis(methylpropylcyclopentadienyl)zirconium dichloride;

[0094] bis(di-n-butylcyclopentadienyl)zirconium dichloride;

[0095] bis(di-n-butylcyclopentadienyl)hafnium dichloride;

[0096] bis(methylbutylcyclopentadienyl)zirconium bis(methanesulfonate);

[0097] bis(di-trimethylsilylcyclopentadienyl)zirconium dichloride;

[0098] bis(di-n-butylcyclopentadienyl)hafnium monochloride monohydride;

[0099] bis(di-n-butylcyclopentadienyl)zirconium monochloridemonohydride;

[0100] bis(dimethylcyclopentadienyl)hafnium dichloride;

[0101] bis(dimethylcyclopentadienyl)dimethylhafnium;

[0102] bis(di-n-propylcyclopentadienyl)zirconium dichloride;

[0103] bis(di-n-propylcyclopentadienyl)zirconium dimethyl;

[0104] bis(1,3-methyl-butyl-cyclopentadienyl)zirconium dichloride; and

[0105] bis(1,3-methyl-butyl-cyclopentadienyl)zirconium dimethyl

[0106] Of these, bis(1,3-dimethylcyclopentadienyl)zirconium dichlorideand bis(1,3-diethylcyclopentadienyl) zirconium dichloride are preferredmetallocene compounds for use in the present invention.

[0107] Of course, mixtures of metallocene compounds satisfying the aboverequirements can also be used. Any metallocene compound that can be usedalone can also be used in conjunction with other suitable metallocenecompounds. Moreover, the amount of metallocene compound used is suchthat it results in the desired ratio of HMW polymer component to LMWpolymer component in the ethylene/α-olefin copolymer with a bimodal MWDto be produced, the ratio in turn being at least in part determined bythe atomic ratio of metal of the non-metallocene transition metalcompound to metal of the metallocene, compound. Generally, the atomicratio is at least 1:1, or at least 2:1, or at least 3:1, or at least4:1, and not higher than 30:1, or not higher than 15:1, or not higherthan 10:1.

[0108] Incorporation of the metallocene catalyst component into thecarrier can be accomplished in various ways. Incorporation of either orboth (a preferably co-employed aluminoxane activator) and themetallocene compound can be into a slurry of catalyst intermediate in anon-polar solvent. The aluminoxane and metallocene compound can be addedin any order, or together, such as as solution in an aromatic or thesame non-polar solvent, to that slurry or to the isolated catalystintermediate. A preferred way of combining aluminoxane and metalloceneis to add a solution of these two components in an aromatic solvent suchas toluene to a slurry of the catalyst intermediate in a differentnon-polar solvent. This addition is preferably done at room temperature,but higher temperatures can also be used as long as the stability of thevarious materials present is not affected. Following the addition, theresulting mixture is usually stirred, preferably at room temperature,for sufficient time to allow all of the components to react and/orinteract substantially completely with each other. Generally theresulting mixture is stirred for at least 0.5 hours, or at least 1.0hours. Thereafter, the liquid phase can be evaporated from the slurry toisolate a free-flowing powder containing both non-metallocene andmetallocene transition metal components. Filtering is usually avoided tosubstantially eliminate the loss of catalytic components. If evaporationof the liquid phase under atmospheric pressure would requiretemperatures that might adversely affect the catalyst components by, forexample, degradation, reduced pressure may be used.

[0109] Preferably, the catalyst intermediate is first recovered from theslurry in the initially employed non-polar solvent or solvent mixture,such as by filtration and/or distilling the solvent, and is thenreslurried in the same or a different non-polar solvent. Non-limitingexamples of suitable non-polar solvents for reslurrying of catalystintermediate include, aliphatic, cycloaliphatic and aromatichydrocarbons such as those set forth above for use in the preparation ofthe initial slurry of the support material in a non-polar solvent, e.g.,n-pentane, isopentane, n-hexane, methylcyclopentane, isohexanes,cyclohexane, n-heptane, methylcyclohexane, isoheptanes, benzene,toluene, ethylbenzene, xylenes and mixtures of two or more thereof.

[0110] Aluminoxanes are preferably employed as activator for themetallocene component of the bimetallic catalyst according to thepresent invention. Suitable aluminoxanes include, but are not limitedto, oligomeric linear and/or cyclic alkylaluminoxanes of the generalformula R—(Al(R)—O)_(n)—AlR₂ for oligomeric, linear aluminoxanes and(—Al(R)—O—)_(m) for oligomeric cyclic aluminoxanes, wherein n is 1-40 or10-20, m is 3-40 or 3-20, and R is a C₁-C₈ alkyl group, and preferablymethyl to provide methylaluminoxane (MAO). MAO is a mixture of oligomerswith a very wide distribution of molecular weights and usually with anaverage molecular weight of about 1200. MAO is typically kept insolution in toluene. It is also possible to use, for the presentpurpose, aluminoxanes of the type just described wherein the alkylgroups in the above general formulae are different. An example thereofis modified methylaluminoxane (MMAO) wherein in comparison to MAO a partof the methyl groups is replaced by other alkyl groups. Modifiedmethylaluminoxanes are disclosed, for example, in U.S. Pat. No.6,001,766.

[0111] The aluminoxane or mixture of aluminoxanes is employed in anamount which results in sufficient activation of the metallocenetransition metal catalyst component of the bimetallic catalyst. Becausethe metallocene transition metal catalyst component of the bimetalliccatalyst produces the LMW polymer component of the ethylene/α-olefincopolymer, under otherwise identical polymerization conditions theweight fraction of LMW polymer component usually increases withincreasing amount of aluminoxane employed. Generally, the atomic ratioof Al in the aluminoxane to metal in the metallocene compound is atleast 10:1, or at least 50:1, or at least 80:1, and not higher than1,000:1, or not higher than 500: 1, or not higher than 300:1.

[0112] The above catalyst can be used as such (i.e., without anyactivator or cocatalyst) for the production of the copolymers of thepresent invention. However, it is preferred to use an additionalcocatalyst together therewith. The purpose of the additional cocatalystis to control the relative activity of the catalyst components, i.e.,the amount of polymer product produced by each, of the two catalystcomponents and thus the ratio of HMW polymer component to LMW polymercomponent. Consequently, if the latter ratio as afforded by the catalystwithout cocatalyst is acceptable for the intended purpose, a cocatalystneed not be used. Generally, however, it is preferred to use thebimetallic catalyst in combination with a cocatalyst that primarilyactivates the non-metallocene catalyst component to form a catalystcomposition suitable for the production of ethylene/α-olefin copolymerswith a controlled bimodal molecular weight distribution in a singlereactor. Suitable cocatalysts are organometallic compounds of Group 1,2, 11, 12 or 13 elements, such as aluminum, sodium, lithium, zinc, boronor magnesium, and in general any one or a combination of any of thematerials commonly employed to activate Ziegler polymerization catalystcomponents. Examples thereof are alkyls, hydrides, alkylhydrides andalkylhalides of the mentioned elements, such as n-butyllithium,diethylzinc, di-n-propylzinc and triethylboron. Usually, however, thecocatalyst will be an alkylaluminum compound, preferably a compound ofthe general formula (III):

R⁵ _(a)AlX_(b)  (III)

[0113] wherein a is 1, 2 or 3, R⁵ is a linear or branched alkyl groupcontaining 1 to 10 carbon atoms, X represents hydrogen atom or halogenatom and b is 0, 1 or 2, provided that the sum (a+b) is 3.

[0114] Preferred types of compounds of formula (III) aretrialkylaluminum, dialkylaluminum hydride, dialkylaluminum halide,alkylaluminum dihydride and alkylaluminum dihalide. The halidepreferably is Cl and/or Br. Preferred alkyl groups are linear orbranched and contain 1 to 6 carbon atoms, such as methyl, ethyl, propyl,isopropyl, butyl, isobutyl, straight-chain and branched pentyl and hexylgroups. Specific examples of suitable cocatalysts are trimethylaluminum,triethylaluminum, tripropylaluminum, triisopropylaluminum,tributylaluminum, triisobutylaluminum, trihexylaluminum,trioctylaluminum, diisobutylhexylaluminum, isobutyldihexylaluminum,diisobutylaluminum hydride, dihexylaluminum hydride, diethylaluminumchloride, and diisobutylaluminum chloride. A preferred cocatalyst istrimethylaluminum (TMA). Other alkylaluminum compounds, for examplethose wherein X in formula (III) is alkoxy having 1 to 6 carbon atomsmay also be used.

[0115] The amount of cocatalyst is sufficient to further activate thenon-metallocene transition metal component (Ziegler component) of thecatalyst. A suitable amount can be determined by one skilled in the art.If too little cocatalyst is used, the catalyst may not be completelyactivated, leading to wasted non-metallocene transition metal componentof the catalyst and also failing to provide the target ratio of HMWpolymer component to LMW polymer component in the copolymer to beproduced. Too much cocatalyst, on the other hand, results in wastedcocatalyst, and may even be an unacceptable impurity in the copolymerproduced. Generally, the amount of cocatalyst used is based on theamount of ethylene fed to the polymerization process. The amount ofcocatalyst generally is at least 5 ppm, or at least 20 ppm, or at least40 ppm, and not higher than 500 ppm, or not higher than 400 ppm, or nothigher than 300 ppm, based on the amount of ethylene used.

[0116] 5.2 Polymerization

[0117] The above-described catalyst or catalyst composition is used tocopolymerize ethylene and one or more α-olefins. Examples of suitableα-olefins include propylene, 1-butene, 1-pentene, 1-hexene,4-methyl-1-pentene, 1-heptene and 1-octene, preferably 1-butene,1-pentene, 1-hexene or 4-methyl-1-pentene and most preferably 1-hexene.The polymerization may be carried out using any suitable, conventionalolefin polymerization process, such as slurry, solution or gas phasepolymerization, but preferably it is carried out in a slurry reactor orin a gas phase reactor, particularly a fluidized-bed reactor. Thepolymerization can be carried out batchwise, semicontinuously orcontinuously. The reaction is conducted in the substantial absence ofcatalyst poisons, such as moisture, carbon monoxide and acetylene, witha catalytically effective amount of the catalyst (composition) attemperature and pressure conditions sufficient to initiate thepolymerization reaction. Particularly desirable methods for producingthe copolymers of the present invention are in a slurry or fluidized bedreactor. Such reactors and processes are described in U.S. Pat. Nos.4,001,382, 4,302,566, and 4,481,301. The polymer produced in suchreactors contains deactivated catalyst particles, because the catalystis not separated from the polymer.

[0118] With the above catalysts, molecular weight of the polymer may besuitably controlled in a known manner, such as by using hydrogen.Hydrogen acts as chain transfer agent. Other reaction conditions beingthe same, a greater amount of hydrogen results in a lower averagemolecular weight of the polymer. The molar ratio of hydrogen/ethyleneused can vary depending on the desired average molecular weight of thepolymer, and can be determined by one skilled in the art for eachparticular instance. Without limiting the present invention, the amountof hydrogen will generally be between 0 and 2.0 moles of hydrogen permole of ethylene.

[0119] Polymerization temperature and time can be determined by oneskilled in the art based on a number of factors, such as the type ofpolymerization process to be used and the type of polymer to beprepared.

[0120] As chemical reactions generally proceed at a greater rate withhigher temperature, polymerization temperature should be high enough toobtain an acceptable polymerization rate. In general, therefore,polymerization temperatures are higher than 30° C., more often higherthan 75° C., and not so high as to cause deterioration of catalyst orpolymer. Specifically, with respect to a fluidized-bed process, thereaction temperature is preferably not so high as to lead to sinteringof polymer particles. In general, polymerization temperatures are lessthan 300° C., or less than 115° C., or less than 105° C.

[0121] The polymerization temperature used in the process is in partdetermined by the density of the ethylene copolymer to be produced. Morein particular, the melting point of the resin depends on resin density.The higher the density of the resin, the higher its melting point. Thecopolymers of the present invention with their relatively high densitiesare preferably made at a temperature above 80° C., but preferably below115° C.

[0122] When a fluidized-bed reactor is used, one skilled in the art canreadily determine appropriate pressures to use. Fluidized-bed reactorscan be operated at pressures of up to about 1000 psi (6.9 MPa) or more,and are generally operated at pressures below 350 psi (2.4 MPa).Preferably, fluidized-bed reactors are operated at pressures above 150psi (1.0 MPa). As is known in the art, operation at higher pressuresfavors heat transfer because an increase in pressure increases the unitvolume heat capacity of the gas.

[0123] Once the catalyst is activated, the activated catalyst has alimited lifetime before it becomes deactivated. As is known to thoseskilled in the art, the half-life of an activated catalyst depends on anumber of factors, such as the species of catalyst (and cocatalyst), thepresence of impurities such as water and oxygen in the reaction vessel,and other factors. An appropriate length of time for carrying out apolymerization can readily be determined by those skilled in the art foreach particular situation.

[0124] The density of ethylene copolymers is in part determined by theamount of comonomer in the polymer. The amount of comonomer needed toachieve this result will depend on the particular comonomer being used.Further, the intended comonomers have different reactivity rates,relative to the reactivity rate of ethylene, with respect to thecopolymerization thereof with the catalysts of the present invention.Therefore the amount of comonomer fed to the reactor will also varydepending on the reactivity of the comonomer.

[0125] In general, the ethylene/α-olefin copolymers of the presentinvention are preferably extruded or injection or blow molded intoarticles or extruded or blown into films. For example, films can beproduced which are about 0.2 to 5.0 mils (5-125 μm), preferably 0.5 to2.0 mils (10-50 μm) in thickness. Blow molded articles include bottles,containers, fuel tanks and drums. The wall thickness of the blow moldedarticles will usually be in the range from about 0.5 to about 2,000 mils(10-50,000 μm).

[0126] The present copolymers may be combined with various additivesconventionally added to polymer compositions, such as lubricants,fillers, stabilizers, antioxidants, compatibilizers, pigments, etc. Manyadditives can be used to stabilize the products. For example, additivepackages including hindered phenols, phosphites, antistats andstearates, for addition to resin powders, can be used for pelletization.

6. EXAMPLES

[0127] The following Examples further illustrate the essential featuresof the present invention. However, it will be apparent to those skilledin the art that the specific reactants and reaction conditions used inthe Examples do not limit the scope of the present invention.

[0128] The properties of the polymers produced in the Examples weredetermined as follows:

[0129] Analysis of the Resin Produced

[0130] Prior to testing, the polymers were processed as follows. 1000ppm each of Irganox™ 1010 (hindered phenol antioxidant) and Irgafos™ 168(phosphite antioxidant), both produced by CK Witco Corp., and 500 ppmAS900 (antistatic amine agent manufactured by Ciba-Geigy, Switzerland),were dry blended with the granular resin. The mixture was then meltmixed on a Brabender twin screw compounder (¾″ screw diameter) at melttemperatures of less than 200° C., with a nitrogen purge to the feedthroat.

[0131] The Flow Index (FI) or Melt Flow Rate I₂₁, g/10 min, at 190° C.was determined as specified in ASTM D 1238 using a load of 21.6 kg.

[0132] The density (g/cm³) was determined as specified in ASTM D 1505-68with the exception that the density measurement was taken after 4 hoursinstead of after 24 hours of conditioning in the density column.

[0133] The molecular weight characterization was performed on a Waters150C gel permeation chromatograph. The chromatograms were run at 140°C., using trichlorobenzene as the solvent. The Waters 150C determinesmolecular weight distribution using the technique of molecular sizeexclusion. This molecular weight data was used to determine the numberaverage molecular weight (Mn), and the weight average molecular weight(Mw).

[0134] Environmental stress cracking resistance (ESCR) was determined byforming an article from the resin to be tested. The time to form crackswas then determined in the environment in question. The resins weretested by using the Bent Strip ESCR test. The ESCR of bottles made fromthe resins was also determined.

[0135] The Bent Strip ESCR was determined according to ASTM D1693condition B as follows. A plaque of the resin was compression molded.Specimens were punched from the plaque. These specimens were notchedacross their broad face, bent in a controlled manner and held in a 10%Igepal™ solution (aggressive soap) at 50° C. until failure. Multiplespecimens were tested and the 50% probability of failure was determined.

[0136] The comonomer distribution of the samples was determined by GelPermeation Chromatography—Fourier Transform IR spectroscopy (GPC-FTIR).The technique of GPC-FTIR for polymer analysis is described in detail inJames N. Willis and L. Wheeler, Applied Spectroscopy, 50, 3, (1996), theentire disclosure of which is incorporated herein by reference. In thistechnique, branch (—CH₃) content is measured as a function of molecularweight of the resin. A stream from GPC, which separates the polyethyleneaccording to its molecular weight, is diverted to an apparatus calledLC-Transform. LC-Transform is an interface between GPC and FTIR. Itsprays the GPC stream onto a disc to form a thin layer of polyethylene.This layer essentially is a fingerprint of GPC chromatogram and also hasthe branching information. In order to extract the branchinginformation, this thin layer is then analyzed using the FTIR. Typicallythe methyl deformation band at 1377 cm⁻¹ is used to measure the shortchain branching in polyethylene. In order to correct for the path lengthfor the IR beam (thickness of the film deposit), the absorbance at 1368cm⁻¹ was also measured. The ratio of absorbances at 1377 cm⁻¹ to 1368cm⁻¹ was used as a normalized measure of the —CH₃ content. Also at lowermolecular weights (<10,000), one would also have chain ends contributingto the IR absorbance at the above peaks. In order estimate thecontribution of chain ends, GPC-FTIR of a low molecular weighthomopolymer was also performed. The latter allowed to measure the —CH₃concentration due to the low molecular weight chain ends.

Comparative Catalyst Preparation Example 1

[0137] The catalyst was prepared in a two-step process.

[0138] STEP 1

[0139] Under an inert atmosphere of dry nitrogen, Davison grade 955silica (367 g), previously calcined at 600° C. under dry nitrogen, andisohexane (3600 mL) were added to a 2 gallon (8 L) glass vesselcontaining a stirring paddle. The stirring rate was set to 100 rpm, andthe temperature of the silica/isohexane slurry was raised to 51-54° C.for the following reagent addition and drying steps. Next,dibutylmagnesium (0.264 mol, 265.3 g of a 2.42 wt % Mg solution inheptane) was added to the stirred silica slurry. After stirring for 2hours, 1-butanol (0.251 mol, 18.6 g) was added to the stirred reactionmixture. After stirring for another 2 hours, titanium tetrachloride(0.160 mol, 30.3 g) was added to the stirred reaction mixture, andstirring was continued for 2 hours. The liquid phase was then removed byevaporation under nitrogen purge, to yield a free flowing powder.

[0140] STEP 2

[0141] Formulation: 6.8 mmol MAO/g of Ti component, Al/Zr=120/1

[0142] Under an inert atmosphere of dry nitrogen, 374 g of thetitanium-containing catalyst component described in Step 1 above, andisopentane (1870 mL) were added to a 2 gallon (8 L) glass vesselcontaining a stirring paddle. The stirring rate was set to 110 rpm. Asolution was prepared by mixing (n-BuCp)₂ZrCl₂(bis(n-butylcyclopentadienyl) zirconium dichloride) (21.2 mmol, 8.564 g)and methylaluminoxane (2.546 mmol, 512.7 g of a 13.4 wt % Al solution intoluene) in a stainless steel Hoke bomb at ambient temperature, under aninert atmosphere of dry nitrogen. This solution was then added to thestirred titanium component/isopentane slurry at ambient temperature,over a period of 50 minutes. The temperature of the reaction mixture wasraised to 47° C., and the liquid phase was removed by evaporation undernitrogen purge to yield a free flowing brown powder.

Catalyst Preparation Example 1

[0143] The catalyst was prepared in a two-step process.

[0144] STEP 1

[0145] Under an inert atmosphere of dry nitrogen, PQ Corporation gradeMS3030 silica (15 g), previously calcined at 700° C. under dry nitrogen,and isohexane (150 mL) were added to a 0.5 L round-bottom flask fittedwith a paddle stirrer. The flask was placed in a 50° C. oil bath, andthe slurry was stirred vigorously. A solution of DBM (dibutylmagnesium)(10.8 mmol, 9.15 g of a 2.85 wt % Mg solution of DBM in heptane) wasfurther diluted with 15 mL of isohexane, and added to a Schlenk flaskcontaining a magnetic stirring bar. This flask was placed in an oil bathat 50° C. and stirred vigorously. Triethylsilanol (10.2 mmol, 1.35 g ofEt₃SiOH) was then added dropwise to the stirred DBM solution at 50° C.After stirring at 50° C. for another 15 minutes, the DBM/triethylsilanolreaction mixture was then added dropwise to the stirred silica slurry at50° C. A solution of titanium tetrachloride (7.5 mmol, 1.4 g) in 10 mLisohexane was added to a Schlenk flask containing a magnetic stirringbar. 1-Pentanol (3.0 mmol, 2.8 mL) was then added dropwise to thestirred TiCI₄ solution at ambient temperature, with a nitrogen purgethrough the vessel to sweep out HCI byproduct. One hour after adding theDBM/triethylsilanol reaction mixture to the stirred silica slurry, theTiCl₄/1-pentanol reaction mixture was added dropwise to thesilica/DBM/triethysilanol reaction product, which was stirred at 50° C.during the addition, and for a further 1 hour after the addition wascomplete. The liquid phase was then removed by evaporation undernitrogen purge at 50° C., to yield a free flowing pale brown power.

[0146] STEP 2

[0147] Under an inert atmosphere of dry nitrogen, 2.5 g of thetitanium-containing catalyst component described in Step 1 above, andisohexane (15 mL) were added to a Schlenk flask containing a magneticstirring bar. The flask was placed in a 50° C. oil bath, and the slurrywas stirred vigorously. A solution was prepared by mixing (1,3-Me₂Cp)₂ZrCl₂ (bis(1,3-dimethycyclopentadienyl) zirconium dichloride, 0.41g) and methylaluminoxane (2.8 g of a 13.4 wt % Al solution in toluene)in a serum bottle at ambient temperature, under an inert atmosphere ofdry nitrogen. This solution was then added dropwise to the stirredtitanium component/isohexane slurry, which was kept at 50° C. The liquidphase was then removed by evaporation under nitrogen purge at 50° C. toyield a free flowing brown powder.

Catalyst Preparation Example 2

[0148] The catalyst was prepared in a two-step process.

STEP 1

[0149] Under an inert atmosphere of dry nitrogen, Davison grade 955silica (367 g), previously calcined at 600° C. under dry nitrogen, andisohexane (3600 mL) were added to a 2 gallon (8 L) glass vesselcontaining a stirring paddle. The stirring rate was set to 100 rpm, andthe temperature of the silica/isohexane slurry was raised to 51-54° C.for the following reagent addition and drying steps. Next,dibutylmagnesium (0.264 mol, 265.3 g of a 2.42 wt % Mg solution inheptane) was added to the stirred silica slurry. After stirring at for 2hours, 1-butanol (0.251 mol, 18.6 g) was added to the stirred reactionmixture. After stirring for another 2 hours, titanium tetrachloride(0.160 mol, 30.3 g) was added to the stirred reaction mixture, andstirring was continued for 2 hours. The liquid phase was then removed byevaporation under nitrogen purge, to yield a free flowing powder.

[0150] STEP 2

[0151] Formulation: 6.6 mmol MAO/g of Ti component, Al/Zr=120/1

[0152] Under an inert atmosphere of dry nitrogen, 359 g of thetitanium-containing catalyst component described in Step 1 above, andisopentane (1860 mL) were added to a 2 gallon (8 L) glass vesselcontaining a stirring paddle. The stirring rate was set to 100 rpm. Asolution was prepared by mixing (1,3-Me₂ Cp)₂ZrCl₂(bis(1,3-dimethylcyclopentadienyl) zirconium dichloride) (19.8 mmol,6.89 g) and methylaluminoxane (2.37 mmol, 471 g of a 13.6 wt % Alsolution in toluene) in a stainless steel Hoke bomb at ambienttemperature, under an inert atmosphere of dry nitrogen. This solutionwas then added to the stirred titanium component/isopentane slurry atambient temperature, over a period of 55 minutes. The temperature of thereaction mixture was raised at 47° C., and the liquid phase was removedby evaporation under nitrogen purge to yield a free flowing brownpowder.

Catalyst Preparation Example 3

[0153] The catalyst was prepared in a two-step process.

STEP 1

[0154] Under an inert atmosphere of dry nitrogen, Crosfield grade ES70silica (416 g), previously calcined at 600° C. under dry nitrogen, andisopentane (2080 mL) were added to a 2 gallon (8 L)vessel containing astirring paddle. The stirring rate was set to 150 rpm, and thetemperature of the silica/isopentane slurry was raised to 49-57° C. forthe following reagent addition and drying steps. Next, dibutylmagnesium(0.298 mol, 258 g of a 2.81 wt % Mg solution in heptane) was added tothe stirred silica slurry. After stirring for 2 hours, titaniumtetrachloride (0.300 mol, 57.0 g) was added to the stirred reactionmixture, and stirring was continued for 1.5 hours. The liquid phase wasthen removed by evaporation under nitrogen purge, to yield a freeflowing powder.

STEP 2

[0155] Under an inert atmosphere of dry nitrogen, 375 g of thetitanium-containing catalyst component described in Step 1 above, andisopentane (1875 mL) were added to a 2 gallon (8 L) glass vesselcontaining a stirring paddle. The stirring rate was set to 100 rpm. Asolution was prepared by mixing (1,3-Me₂ Cp)₂ZrCl₂(bis(1,3-dimethylcyclopentadienyl) zirconium dichloride) (21.9 mmol,7.62 g) and methylaluminoxane (2.62 mmol, 524 g of a 13.4 wt % Alsolution in toluene) in a stainless steel Hoke bomb at ambienttemperature, under an inert atmosphere of dry nitrogen. This solutionwas then added to the stirred titanium component/isopentane slurry atambient temperature, over a period of 30 minutes. The temperature of thereaction mixture was raised to 48° C., and the liquid phase was removedby evaporation under nitrogen purge to yield a free flowing brownpowder.

Comparative Polymerization Example 1

[0156] The polymerization was carried out in a gas phase reactor whichwas run at 100.0° C., 356 psig (2.45 MPa) total reactor pressure, andwith the following partial pressures: 162 psi (1.12 MPa) ethylene, 28.0psi (193 kPa) isopentane, 0.81 psi (5.6 kPa) 1-hexene and 2.4 psi (17kPa) hydrogen. The molar gas ratios were 0.0050:1 1-hexene:ethylene and0.0149:1 hydrogen:ethylene with a residence time of 2.67 hr. Thecocatalyst trimethylaluminum (TMA) level was 128 ppm by weight and thewater addback level was 34 ppm by volume. The ppm values are based onethylene feed. Catalyst as described in the Comparative CatalystPreparation Example was fed to the reactor. 140 pounds (64 kg) of resinwere collected for sampling.

Polymerization Example 1

[0157] A 3.8 L stainless steel autoclave, equipped with a paddlestirrer, and under a slow nitrogen purge at 50° C. with stirring set to300 rpm, was charged with 1500 mL of dry heptane, 40 μL of water, 4.2mmol (3.0 mL of a 1.4 Molar solution in heptane) of trimethylaluminum(TMA), and 60 mL of 1-hexene. The reactor was then closed and thestirring speed set for 900 rpm, and the internal temperature was raisedto 95° C., then the internal pressure was raised from 10 psi (69 kPa) to16 psi (110 kPa) by addition of 6 psi (40 kPa) of hydrogen. Ethylene wasthen introduced into the reactor and the internal pressure was increasedto 226 psi (1.56 MPa). Finally, 0.0542 g of the catalyst prepared asdescribed in Catalyst Preparation Example 1 was added to the autoclave.The reactor pressure was maintained at 220-225 psi (1.52-1.55 MPa) for30 minutes by addition of ethylene, after which time the ethylene flowto the reactor was stopped and the reactor was cooled to roomtemperature and vented to the atmosphere. The contents of the autoclavewere removed, and all solvents were removed from the product byevaporation, to yield 132.5 g of polyethylene resin (ethylene/1-hexenecopolymer).

[0158] Addition of very small amounts of water to a polymerizationreactor containing TMA (or any other alkylaluminum compound)significantly increases the activity of the metallocene catalystcomponent relative to the non-metallocene catalyst component. This wateraddition process is commonly referred to as “water addback.” Hence,water addback is a method of controlling the weight fractions of the HMWand LMW polymer components. This is an extremely important technique ina commercial reactor to produce the target polyethylene resin. Forexample, if the product must contain 60% by weight HMW polymer componentand 40% by weight LMW polymer component, water addback is normally usedto meet this product composition requirement. U.S. Pat. No. 5,525,678discloses this water addback technique for controlling polymer weightfractions with a bimetallic catalyst.

Polymerization Example 2

[0159] A 3.8 L stainless steel autoclave, equipped with a paddlestirrer, and under a slow nitrogen purge at 50° C. with stirring set to300 rpm, was charged with 1500 mL of dry heptane, 20 μL of water, 2.8mmol (2.0 mL of a 1.4 molar solution in heptane) of trimethylaluminum(TMA), and 60 mL of 1-hexene. The reactor was then closed and thestirring speed set for 900 rpm, and the internal temperature was raisedto 95° C., then the internal pressure was raised from 10 psi (69 kPa) to16 psi (110 kPa) by addition of 6 psi (40 kPa) of hydrogen. Ethylene wasthen introduced into the reactor and the internal pressure was increasedto 226 psi (1.56 MPa). Finally, 0.0507 g of the catalyst described inCatalyst Preparation Example 2 was added to the autoclave. The reactorpressure was maintained at 220-225 psi (1.52-1.55 MPa) for 30 minutes byaddition of ethylene, after which time the ethylene flow to the reactorwas stopped and the reactor was cooled to room temperature and vented tothe atmosphere. The contents of the autoclave were removed, and allsolvents were removed from the product by evaporation, to yield 71.9 gof polyethylene resin (ethylene/1-hexene copolymer).

Polymerization Example 3

[0160] A 3.8 L stainless steel autoclave, equipped with a paddlestirrer, and under a slow nitrogen purge at 50° C. with stirring set to300 rpm, was charged with 1500 mL of dry heptane, 40 μL of water, 4.2mmol (3.0 mL of a 1.4 molar solution in heptane) of trimethylaluminum(TMA) and 60 mL of 1-hexene. The reactor was then closed and thestirring speed set for 900 rpm, and the internal temperature was raisedto 100° C., then the internal pressure was raised from 12 psi (83 kPa)to 20 psi (140 kPa) by addition of 8 psi (6 kPa) of hydrogen. Ethylenewas then introduced into the reactor and the internal pressure wasincreased to 225 psi (1.55 MPa). Finally, 0.0443 g of the catalystdescribed in Catalyst Preparation Example 3 was added to the autoclave.The reactor pressure was maintained at 220-225 psi (1.52-1.55 Mpa) for60 minutes by addition of ethylene, after which time the ethylene flowto the reactor was stopped and the reactor was cooled to roomtemperature and vented to the atmosphere. The contents of the autoclavewere removed, and all solvents were removed from the product byevaporation, to yield 80.7 g of polyethylene resin (ethylene/1-hexenecopolymer).

[0161] Table 1 summarizes some of the properties of the resins preparedaccording to the Comparative Polymerization Example and PolymerizationExamples 1-3. In addition, properties of some commercially availableresins (Sample A to G) are also shown. TABLE 1 Resin Catalyst Melt Exam-Prep. Flow Rate ple Example Reactor Catalyst I₂₁ (g/ Density ESCR No.No. Type Type 10 min) (g/cm³) (hr) Comp. Comp. single bimetallic 170.959 125 * * single bimetallic 26 0.960 128 1 1 single bimetallic 280.957 454 2 2 single bimetallic 24 0.959 436 3 3 single bimetallic 320.960 335 A¹ N/A single Cr 31 0.954 24 B² N/A single Cr 41 0.954 21 C³N/A single Cr 22 0.955 47 D⁴ N/A single Cr 21 0.954 61 E⁵ N/A tandemZiegler 31 0.959 134 F⁶ N/A tandem Ziegler 30 0.957 80 G⁷ N/A tandemZiegler 25 0.954 372

[0162] It is well known that lowering the density of linear polyethyleneresins increases the ESCR of the resin (see, e.g., Constant D. R. andBerg B. R., Blow Molding Retec '97, Oct. 1-3, 1997, Industrial MaterialsInstitute, National Research Council, Canada, Conference Proceedings, p.236). Analysis of the data in Table 1 shows the ESCR performance of theresins of the polymerization examples according to the presentinvention, is significantly better than that of either commercial singleor tandem reactor resins or the single reactor resin made withbimetallic catalyst according to U.S. Pat. No. 5,539,076. Comparing theperformance of the commercial resins A-D to E-G shows that tandem resinshave a significant advantage over conventional single reactor resins.The ESCR of examples E, F, and G is greater than the ESCR of examples Athrough D, even though examples E through G have higher densities.Increasing the densities of examples A-D to 0.959 would further lowertheir ESCR significantly. Commercial tandem grades are known to have asuperior ESCR/density balance compared to resins produced usingconventional single reactor systems.

[0163] Examples E, F, and G and the comparative examples have similarESCR/density performance. In addition the comparative examples show thatthe performance of resins produced using the technology given in U.S.Pat. No. 5,539,076 is reproducible, and that the improvements shown inExamples 1, 2, and 3 are not inherent in the technology given in U.S.Pat. No. 5,539,076.

[0164] The ESCR performance of the resins of the polymerization examplesaccording to the present invention (Examples 1, 2, and 3) is two to fourtimes better than that of both the commercial tandem and the existingsingle reactor Ti/Zr resins (examples E and F and comparative examples).This improvement in performance is at similar resin density.

[0165] The densities of the resins of Polymerization Examples 1, 2, and3 according to the present invention are 0.004 to 0.005 units higherthan those of the conventional single reactor resins (examples A, B, Cand D). Notwithstanding this increase in density, the ESCR performanceof the resins of Polymerization Examples 1, 2, and 3 is more than fivetimes better than that of commercial single reactor resins (Examples Ato D).

[0166]FIG. 1 shows the Density versus Bent Strip ESCR for commercialresins and those according to the present invention. The resinsaccording to the present invention are clearly superior to commercialresins and resins produced according to U.S. Pat. No. 5,539,076(Comparative Examples).

[0167]FIG. 2 is a plot of comonomer distribution (Branching Content,B.C.) as a function of molecular weight (from GPC-FTIR measurements asdescribed above) for bimodal MWD resin produced using the Ti/Zrbimetallic catalyst technology described in U.S. Pat. No. 5,539,076. Itis apparent that most of the comonomer (branches) is in the LMW polymercomponent of the bimodal resin. The HMW polymer component contains onlya very low level of comonomer (branches). FIG. 2 shows that bimodalresins produced using the Ti/Zr bimetallic catalyst technology in asingle reactor are not expected to meet the requirement disclosed inBailey et al. for the production of high ESCR PE resins at high density.

[0168]FIG. 3 is a plot of comonomer distribution (Branching Content,B.C.) as a function of molecular weight (from GPC-FTIR measurements asdescribed above) for bimodal MWD resin according to the presentinvention, produced with a catalyst as described in Catalyst PreparationExample 1. It is apparent that the comonomer (branches) is much moreevenly distributed between the HMW polymer component and the LMW polymercomponent than in the bimodal resin of FIG. 2. FIG. 3 shows that bimodalresins produced using the Ti/Zr bimetallic catalyst technology in asingle reactor can indeed meet the requirement disclosed in Bailey etal. for the production of high ESCR PE resins at high density.

[0169] Certain features of the present invention are described in termsof a set of numerical upper limits and a set of numerical lower limits.It should be appreciated that ranges from any lower limit to any upperlimit are within the scope of the invention unless otherwise indicated.

[0170] All patents, test procedures, and other documents cited in thisapplication are fully incorporated by reference to the extent suchdisclosure is not inconsistent with this application and for alljurisdictions in which such incorporation is permitted.

What is claimed is:
 1. An ethylene/α-olefin copolymer having a densityof at least 0.953 g/cm³ and a Bent Strip ESCR, T₅₀, of at least 175hours, the copolymer prepared in a single reactor.
 2. The copolymer ofclaim 1, wherein the copolymer has a density of at least 0.955 g/cm³. 3.The copolymer of claim 2, wherein the copolymer has a T₅₀ of at least200 hours.
 4. The copolymer of claim 1, wherein the copolymer has adensity of at least 0.957 g/cm³.
 5. The copolymer of claim 4, whereinthe copolymer has a T₅₀ of at least 200 hours.
 6. The copolymer of claim2, wherein the copolymer has a T₅₀ of at least 250 hours.
 7. Thecopolymer of claim 5, wherein the copolymer has a T₅₀ of at least 300hours.
 8. The copolymer of claim 7, wherein the copolymer has a T₅₀ ofat least 350 hours.
 9. The copolymer of claim 6, wherein the copolymerhas a density of at least 0.959 g/cm³.
 10. The copolymer of claim 7,wherein the copolymer has a density of at least 0.959 g/cm³.
 11. Thecopolymer of claim 8, wherein the copolymer has a density of at least0.959 g/cm³.
 12. The copolymer of claim 8, wherein the copolymer has aT₅₀ of at least 400 hours.
 13. The copolymer of claim 2, wherein thecopolymer has a Melt Flow Rate I₂₁ of at least 20 g/10 min.
 14. Thecopolymer of claim 3, wherein the copolymer has a Melt Flow Rate I₂₁ ofat least 22 g/10 min.
 15. The copolymer of claim 4, wherein thecopolymer has a Melt Flow Rate I₂₁ of at least 24 g/10 min.
 16. Thecopolymer of claim 13, wherein the copolymer has a Melt Flow RatioI₂₁/I₂ of at least
 100. 17. The copolymer of claim 1, wherein thecopolymer has a Melt Flow Ratio I₂₁/I₂ of at least
 120. 18. Anethylene/α-olefin copolymer, wherein the copolymer has a bimodalmolecular weight distribution and comprises at least a high molecularweight (HMW) polymer component and at least a low molecular weight (LMW)polymer component which has a lower average molecular weight and ahigher density than the HMW polymer component, the copolymer prepared ina single reactor with a bimetallic polymerization catalyst comprising aZiegler component and a metallocene component.
 19. The copolymer ofclaim 18, wherein the density of the HMW polymer component is in a rangefrom 0.930 g/cm³ to 0.950 g/cm³.
 20. The copolymer of claim 19, whereinthe density of the LMW polymer component is at least 0.955 g/cm³. 21.The copolymer of claim 19, wherein the density of the copolymer is atleast 0.954 g/cm³.
 22. The copolymer of claim 20, wherein the HMWpolymer component has a molecular weight distribution, M_(w)/M_(n) offrom 3 to
 6. 23. The copolymer of claim 22, wherein the LMW polymercomponent has a M_(w)/M_(n) of not higher than
 6. 24. The copolymer ofclaim 18, wherein the weight ratio of HMW polymer component to LMWpolymer component is from 60:40 to 40:60.
 25. The copolymer of claim 18,wherein the copolymer comprises units derived from one or more α-olefinscontaining 3 to 10 carbon atoms.
 26. The copolymer of claim 1, whereinthe copolymer comprises units derived from one or more α-olefinsselected from the group consisting of 1-butene, 1-pentene,4-methyl-1-pentene, 1-hexene and 1-octene.
 27. The copolymer of claim18, wherein the α-olefin comprises 1-hexene.
 28. The copolymer of claim18, wherein the copolymer comprises 0.5 to 20 mol % of units derivedfrom one or more α-olefins.
 29. The copolymer of claim 26, wherein thecopolymer comprises 1 to 10 mol % of units derived from one or moreα-olefins.
 30. The copolymer of claim 18, wherein the Ziegler componentof the polymerization catalyst comprises at least one of titanium andvanadium.
 31. The copolymer of claim 30, wherein the bimetallic catalystcomprises titanium and zirconium.
 32. The copolymer of claim 18, whereinthe metallocene component of the polymerization catalyst compriseszirconium.
 33. An ethylene/α-olefin copolymer having a bimodal molecularweight distribution and a density of at least 0.953 g/cm³, the copolymercomprising at least a HMW polymer component and at least a LMW polymercomponent having a lower average molecular weight than the HMW polymercomponent, the HMW polymer component comprising at least 30 mol % of thetotal α-olefin present in the copolymer, the copolymer prepared in asingle reactor.
 34. The copolymer of claim 33, wherein the copolymer isproduced by a bimetallic polymerization catalyst comprising a Zieglercomponent and a metallocene component.
 35. The copolymer of claim 33,wherein the weight ratio of HMW polymer component to LMW polymercomponent is from 65:35 to 35:65.
 36. The copolymer of claim 34, whereinthe metallocene component comprises at least one dicyclopentadienyltransition metal compound wherein each of the two cyclopentadienyl ringsis independently substituted by up to 5 alkyl groups having 1 to 4carbon atoms, provided that two alkyl substituents on the same ring maybe replaced by an alkylene group and two alkyl substituents on differentrings may be replaced by an alkylene, alkylidene or silicon-containinggroup which forms a bridge between said rings and further provided thatthe total number of substituents on the rings does not exceed
 8. 37. Thecopolymer of claim 34, wherein the metallocene component comprises abis(dialkylcyclopentadienyl) zirconium compound.
 38. A process formaking an ethylene/α-olefin copolymer having a density of at least 0.953g/cm³ and a Bent Strip ESCR, T₅₀, of at least 175 hours in a singlereactor, said process comprising contacting, under polymerizationconditions, ethylene, one or more α-olefins, hydrogen and a bimetallicpolymerization catalyst comprising a Ziegler component and a metallocenecomponent, the combination of Ziegler component and metallocenecomponent being selected to result in a copolymer having at least a HMWpolymer component and at least a LMW polymer component, the HMW polymercomponent having a higher average molecular weight than the LMW polymercomponent and comprising at least 30 mol % of the total α-olefinincorporated into the copolymer.
 39. The process of claim 38, whereinthe one or more α-olefins comprise at least one of 1-butene, 1-pentene,4-methyl-1-pentene, 1-hexene and 1-octene.
 40. The process of claim 38,wherein the copolymer comprises 0.5 to 20 mol % of units derived fromone or more α-olefins containing 3 to 10 carbon atoms.
 41. The processof claim 40, wherein the copolymer is an ethylene/1-hexene copolymer.42. The process of claim 39, wherein the metallocene component comprisesa biscyclopentadienyl zirconium compound.
 43. The process of claim 39,wherein the Ziegler component comprises at least one of titanium andvanadium.
 44. The process of claim 43, wherein the metallocene componentcomprises a bis(dimethylcyclopentadienyl) zirconium compound.
 45. Theprocess of claim 38, wherein the process is carried out in a gas phasereactor.
 46. The process of claim 38, wherein the process is carried outin a slurry reactor.
 47. A method of improving the ESCR of anethylene/α-olefin copolymer having a bimodal molecular weightdistribution and produced in a single reactor, the method comprisingpolymerizing the comonomers in the presence of a bimetallicpolymerization catalyst comprising a Ziegler component and a metallocenecomponent which affords a copolymer having at least a high molecularweight (HMW) component and at least a low molecular weight (LMW)component having a lower average molecular weight than the HMWcomponent, the HMW component comprising at least 30 mol % of the totalα-olefin incorporated into the copolymer.
 48. A blow-molded articlecomprising the copolymer of claim
 1. 49. The blow molded article ofclaim 48, which is a bottle.
 50. An extruded article comprising thecopolymer of claim
 1. 51. The extruded article of claim 50, which is apipe.
 52. A polymerization catalyst for the preparation, in a singlereactor, of an ethylene/α-olefin copolymer having a bimodal molecularweight distribution and a density of at least 0.953 g/cm³ and comprisingat least a high molecular weight (HMW) polymer component and at least alow molecular weight (LMW) polymer component having a lower averagemolecular weight than the HMW polymer component, the HMW polymercomponent comprising at least 30 mol % of the total α-olefin present inthe copolymer, the catalyst comprising a Ziegler component producing theHMW polymer component and a metallocene component producing the LMWpolymer component, the metallocene component comprising twocyclopentadiene rings which have a total of 3 to 8 ring substituents.53. The polymerization catalyst of claim 52, wherein said ringsubstituents are alkyl groups.
 54. The polymerization catalyst of claim52, wherein the metallocene component comprises a total of 4 to 6 ringsubstituents.
 55. The polymerization catalyst of claim 53, wherein themetallocene is a zirconocene.
 56. The polymerization catalyst of claim52, wherein the Ziegler component comprises at least one of titanium andvanadium.
 57. The polymerization catalyst of claim 52, wherein themetallocene component comprisesbis(1,3-dialkylcyclopentadienyl)zirconium dichloride or dimethyl and thealkyl groups are selected from methyl and ethyl.
 58. The polymerizationcatalyst of claim 52, wherein the metallocene component comprisesbis(1,3-dimethylcyclopentadienyl)zirconium dichloride and the Zieglercomponent comprises magnesium and titanium.
 59. A supported bimetallicpolymerization catalyst suitable for use in the production ofethylene/α-olefin copolymer having a bimodal molecular weightdistribution, a density of at least 0.953 g/cm³ and a Bent Strip ESCR,T₅₀, of at least 175 hours in a single reactor, said catalyst comprisinga solid support, at least one non-metallocene transition metal source,at least one metallocene compound and at least one aluminoxane, the atleast one metallocene compound comprising at least onedicyclopentadienyl transition metal compound wherein each of the twocyclopentadienyl rings is independently substituted by up to 5substituents having not more than 4 carbon atoms, provided that twoadjacent substituents on the same ring together with the carbon atoms towhich they are bonded may form a 5- or 6-membered non-aromatic ring andtwo substituents on different rings may be replaced by a C₂-C₄ alkyleneor alkylidene group or silicon-containing group which forms a bridgebetween said rings and further provided that the total number ofsubstituents on the rings does not exceed
 6. 60. The supportedbimetallic catalyst of claim 59, wherein the non-metallocene transitionmetal source comprises at least one compound containing a Group IV or Vtransition metal.
 61. The supported bimetallic catalyst of claim 60,wherein the Group IV or V transition metal is at least one of titaniumand vanadium.
 62. The supported bimetallic catalyst of claim 60, whereinthe non-metallocene transition metal source comprises halogen.
 63. Thesupported bimetallic catalyst of claim 60, wherein the non-metallocenetransition metal source is a tetravalent titanium compound.
 64. Thesupported bimetallic catalyst of claim 59, wherein the support comprisessilica.